There is a technique which stops a rotation of a disk of a hard disk drive which is not used to reduce power consumption of a storage device. This technique stops a rotation of a disk to which an I/O (Input/Output) request does not exist during a predetermined period of time, and when an I/O request occurs to the disk in a state of rotation stop, a rotation of the disk is restarted to be a state in which the I/O is allowed to be performed.
However, when an I/O is performed to a disk in a state of rotation stop, a response of the I/O is not returned for a maximum of several minutes from a point when restarting a rotation of a disk to a point when the rotation of the disk becomes stable. Therefore, in related art, a rotation of a disk has not been stopped to a disk with high I/O frequency such as a disk used for an on-line business system, and a rotation of a disk has been stopped to a disk with low I/O frequency for a batch operation such as backup operation. Thereby, power consumption is reduced by stopping a rotation of a backup destination disk to which an I/O does not occur in a period other than a period of backup operation, and I/O to the disk which has stopped the rotation is performed by a batch operational to hide an influence of the waiting time of the I/O.
On the other hand, there is a technique which reduces power to a disk with high I/O frequency such as a disk used for an on-line business system. Such technique stops a rotation of a part of disks which are made redundant by using RAID (Redundant Arrays of Inexpensive Disks) and uses other disks.
Hereinafter, for the sake of illustration, a disk which is used in a normal operation is referred to as an “active disk”, and a disk which stops rotation is referred to as a “standby disk”. A non-volatile medium which temporarily stores write data while the standby disk stops is referred to as a “cache”.
When a failure occurs on one of active disks which are made in redundant configuration, in order to read the data from a standby disk which is also made in redundant configuration, the standby disk is rotated, data written in the cache is reflected to the standby disk, and the data is read out. However, since it takes time to stabilize the rotation of the standby disk, it takes time to restart reading data.    Patent document 1: Japanese Laid-open Patent Publication No. 2009-187450    Patent document 2: Japanese Laid-open Patent Publication No. 2009-238159    Patent document 3: Japanese Laid-open Patent Publication No. 2009-048360